The processing of a microwave-frequency wave, for example received by a satellite, requires the development of specific components, allowing the propagation, the amplification, and the filtering of this wave.
For example a microwave-frequency wave received by a satellite must be amplified before being returned to the ground. This amplification is possible only by separating the set of frequencies received into channels, each corresponding to a given frequency band. The amplification is then carried out channel by channel. The separation of the channels requires the development of bandpass filters.
The development of satellites and the increased complexity of the signal processing to be performed, for example a reconfiguration of the channels in flight, has led to the need to implement frequency-tunable bandpass filters, that is to say for which it is possible to adjust the central filtering frequency which may otherwise be known as the filter tuning frequency.
One of the known technologies of tunable bandpass filters in the field of microwave-frequency waves is the use of passive semi-conducting components, such as PIN diodes, continuously variable capacitors or capacitive switches. Another technology is the use of MEMS (for micro electromechanical system) of ohmic or capacitive type.
These technologies are complex, consume electrical energy and are rather unreliable. These solutions are also limited in terms of the signal power processed. Moreover, a consequence of frequency tunability is an appreciable degradation in the performance of the filter, such as its quality factor Q.
Moreover, the technology of filters with dielectric resonator is known. It makes it possible to produce non-tunable bandpass filters.
FIG. 1 describes an exemplary non-tunable microwave-frequency wave filter with a dielectric resonator.
An input excitation element 10 introduces the wave into the cavity (input port), this element is typically a conducting medium such as a coaxial cable or a waveguide.
The cavity 13 is a closed cavity consisting of metal, typically aluminum or a metal alloy such as Invar.
An output excitation element 11, typically a conducting medium such as a coaxial cable or a waveguide, makes it possible for the wave to exit the cavity (output port).
The resonator 12 consists of a dielectric element of arbitrary shape, typically round or square, and disposed inside the metallic cavity 13. The dielectric material is typically zirconia, alumina or barium magnesium tantaate (“BMT”).
From an electromagnetic point of view, a resonator is characterized by its resonant frequency, for which a steady, periodic vibration of the electromagnetic field is established.
A bandpass filter allows the propagation of a wave over a certain frequency span and attenuates this wave for the other frequencies. A passband and a central frequency of the filter are thus defined. For frequencies around its central frequency, a bandpass filter exhibits high transmission and low reflection.
A filter comprises at least one resonator, coupled to the ports of the filter, input port and output port.
In order to increase their selectivity, that is to say their capacity to attenuate the signal outside of the passband, these filters can be composed of a plurality of resonators coupled together.
The central frequency and the passband of the filter depend at one and the same time on the individual resonators and on their respective at least one resonant frequency, and on the coupling together of the resonators as well as the couplings to the ports of the filters.
Coupling means are for example openings or slots which may otherwise be known as irises, electrical or magnetic probes or microwave-frequency lines.
The passband of the filter is characterized in various ways according to the nature of the filter.
The parameter S is a parameter which expresses the performance of the filter in terms of reflection and transmission. By numbering the two access ports 1 and 2, S11 corresponds to a measurement of the reflection and S12 or S21 to a measurement of the transmission, respectively.
A filter carries out a filtering function. This function can generally be approximated via mathematical models (iterative functions such as Chebychev, Bessel, functions etc.). These functions are generally based on ratios of polynomials:
For a filter carrying out a filtering function of Chebychev or generalized Chebychev type, the passband of the filter is determined at equi-ripple of S11 (or S22), for example at 15 dB or 20 dB of reduction in the reflection with respect to a frequency that is not within a range of the passband of the filter. For a filter carrying out a function of Bessel type, the frequency band corresponding to a bandwidth of −3 dB (when S21 crosses S11) is determined to be the passband.
FIG. 2 describes an exemplary filter 13 with three resonators 23, 24, 25 coupled together and situated inside 3 cavities coupled through coupling irises. Conducting separation walls 26, 27 separate the resonators, and the coupling irises or openings 21 and 22 couple the resonators together. An input excitation element 10 may introduce a wave into the filter, and an output excitation element 11 may make it possible for the wave to exit the filter 13.
A characteristic example of frequency response (parameters S11 and S12) of a filter is illustrated in FIG. 3. The curve 31 corresponds to the reflection S11 of the wave on the filter as a function of its frequency (f) measured in GHz. The equi-ripple passband at 20 dB (which is marked along the axis dB in the graph of FIG. 3) of reflection is noted with numeral 36. The filter exhibits a central frequency (fc) corresponding to the frequency of the middle of the passband. The curve 32 of FIG. 3 describes the corresponding transmission S12 of the filter as a function of frequency.
The tuning of the filter making it possible to obtain a transmission maxima (reflection minima) for a given frequency band may be a very complicated process and depends on the set of parameters of the filter. It is moreover dependent on temperature and environmental conditions in general.
In order to perform an adjustment of the filter to obtain a precise central frequency of the filter, the resonant frequencies of the resonators of the filter can be very slightly modified with the aid of metallic screws, but this method performed in an empirical manner, is very expensive time-wise and allows only very weak frequency tunability, typically of the order of a few %. In this case, the objective is not tunability but the obtaining of a precise value of the central frequency; and it is desired to obtain a reduced sensitivity of the frequency of each resonator in relation to the depth of the screw.
The circular or square symmetry of the resonators simplifies the design of the filter and the selection of the mode (TE for Transverse Electric or TM for Transverse Magnetic) which propagates in the filter.
U.S. Pat. No. 7,705,694 describes a passband-tunable filter composed of a plurality of dielectric resonators coupled together, of radially non-uniform shape and uniform along an axis z perpendicular to the direction of propagation. Each resonator is able to perform a rotation about the axis z between two positions, which induces a change in the value of the width of the passband, typically from 51 Mhz to 68 Mhz. This device allows tunability as regards the value of the width of the passband of the filter, but not as regards its central frequency.